This invention relates to a sol-gel process for producing dry gel monoliths and, more particularly, to a drying process that provides a dry, crack-free porous gel monolith using elevated subcritical temperatures and pressures.
Sol-gel processes for fabricating high-purity monolithic articles of glass and ceramic are well known. In such processes, a desired solution, or "sol," consisting of glass- or ceramic-forming compounds, solvents, and catalysts is poured into a mold where it is allowed to react. Following hydrolysis and condensation reactions, the sol forms a porous matrix of solids, generally referred to as a "gel." With additional time, the gel shrinks in size as fluid is expelled from its pores. The wet gel is then dried in a controlled environment to remove the remaining fluid from its pores, after which it is densified into a solid monolith.
Sol-gel processes have many advantages, including, for example, chemical purity and homogeneity, flexibility in the selection of compositions, processing at relatively low temperatures, and producing monolithic articles close to their final desired shapes, thereby minimizing finishing costs. Nevertheless, sol-gel processes have generally proven to be difficult to use for producing large monoliths that are free of cracks. Typically, cracks arise during the final drying step of the process, and are believed to result from stresses due to capillary forces in the gel pores. Efforts to eliminate the cracking problem in sol-gel monoliths have been diverse. However, eliminating the cracking problem has also meant sacrificing one or more of the benefits of the process described above.
Known techniques for drying sol-gel derived bodies generally consist of one of two distinctly different approaches. In one approach, the wet gels are dried at ambient pressure (14.7 psia), and at temperatures close to or slightly higher than the boiling point of the solvent used as the drying medium. See, for example, Wang et al., U.S. Pat. No. 5,243,769, which describes just such a processing technique, and the disclosure of which is hereby incorporated by reference herein. One variation of this approach consists of heating the gel to such temperatures in a chamber having several pin holes through which the evaporating liquid escapes. Because the chamber is ventilated to the outside environment, the pressure does not increase above ambient pressure. Although this approach is generally effective, it can be very slow, at times requiring as much as a month or more to complete the drying process. While this slow drying rate can be increased by increasing the area of the pin holes, doing so frequently leads to cracking.
Other variations of this approach have been used in attempts to eliminate cracking during the final drying step. For example, colloidal silica particles have been added to the sol to increase the average pore size and strength of the solid matrix. Although this method is generally effective, the presence of colloidal silica particles sacrifices the gel's otherwise inherent homogeneity, thus restricting the range of compositions that can be utilized. In addition, devitrification spots can be created if mixing of the colloidal silica particles is not perfect. Drying control additives may also be added to the sol as this produces a more uniform pore size distribution, thereby strengthening the gel matrix. These additives, such as dimethyl formamide, are then removed during the drying step. Although generally effective in eliminating cracking, this method has the tendency to produce monoliths having a large number of bubbles. Using different catalysts can also increase the pore size distribution to aid in eliminating cracking during the drying step, but such a method has not proven to be particularly successful for large monoliths since no catalyst has yet been shown to be able to produce average pore sizes above about 100.ANG.. One other variation of this approach has been to hydrothermally age the gel prior to drying. This increases the average pore size in the gel, and correspondingly decreases the capillary stresses encountered during drying. Although this method is generally effective, the aging step increases the time and the equipment costs for drying gels, and thus also increases the cost of the final product.
The second approach for drying sol-gel derived bodies to produce a dry gel monolith is to heat the wet gel above the critical temperature of the solvent being used as the drying medium in a drying chamber that permits the pressure to exceed the solvent's critical pressure. Because there is no vapor/liquid interface in the pores of the gel matrix when the temperature and pressure exceed the critical temperature and pressure of the drying solvent, no capillary force exists. The solvent is removed from the pores while the critical temperature and pressure is exceeded until a dry gel results. This technique is known in the art as "supercritical drying." Although this technique is effective, it requires relatively expensive equipment and can be dangerous.
Generally, sol-gels dried using ambient pressure techniques undergo considerable shrinkage, and the pore sizes of the resulting dry gels are usually relatively small. In contrast, sol-gels dried by high pressure supercritical drying techniques generally experience very little shrinking, which means that the resulting dry gels have relatively larger pore sizes.
Dry gels having larger pore sizes, i.e., having a pore radius of at least 40.ANG., are preferred for the manufacture of near net shape dense monolithic glass or ceramic objects by a sol-gel process. This is due to the fact that to produce dense glass monoliths, it is necessary to heat the porous dry gels to a temperature of at least 1200.degree. C. to remove the pores. This process is known as "sintering." The sintering process usually consists of several sequential steps, such as the removal of physical water, decomposition of chemically bonded hydrocarbon groups by reaction with gaseous oxygen or air, removal of the products of the decomposition by purging with helium, and the final densification of the dried gel under a flow of helium. In a sintering process, it is important that the dry gel has sufficiently large pores so that the reactant gases, whether oxygen or air, purge gases such as helium, and other products of the reaction can pass in and out of the pores relatively easily without getting trapped in the porous gel matrix. For example, it is known that if a gel contains small pores, premature collapse of the pores may result at temperatures as low as 700.degree. C., which is below the glass transition temperature of fused silica, generally about 1200.degree. C. Premature collapse of small pores invariably traps gases. Any trapping or incomplete removal of residual hydrocarbon or hydroxyl groups will cause cracking during the sintering operation due to pressure build-up at higher temperatures. However, if a gel contains pores having a radius of at least 40.ANG., those pores generally remain open even at temperatures above 1200.degree. C. See, "Monolith Formation from the Sol-Gel Process," M. Yamane, Chapter 10 of the book Sol-Gel Technology for Thin Films, Fibers, Preforms, Electronics and Specialty Shapes, edited by Lisa C. Klein, Noyes Publications, 1988. Thus, such larger pore sizes generally act as large diameter channels for the mass transfer of gases in and out of the gel matrix. The larger the pore size of the dry gel, the easier it is to sinter such gels to obtain crack-free monolithic glass pieces of any desired shape or size.
It is apparent from the foregoing discussion that supercritically dried gels, commonly known as "aerogels," should ideally be chosen for fabrication of dense sol-gel monoliths since this approach results in larger pore sizes. Although this is generally true, the supercritical drying process has one serious disadvantage in that it requires operation of an autoclave at higher pressures than the critical pressure of the solvents used as the drying medium. Most of the drying solvents used in a supercritical drying process have high critical pressures. For example, if ethyl alcohol is used as the drying solvent in a supercritical drying step, the supercritical temperature and pressure of operation are 243.degree. C. and 928 psia, respectively. Thus, this type of high pressure operation requires the design and fabrication of specialty equipment. Such equipment can be prohibitively expensive for large scale manufacturing. However, the cost of the drying chamber can be reduced substantially if the operating pressure can be reduced below the critical pressure.
Inorganic solvents, such as liquid carbon dioxide, have also been used as the drying solvent in an attempt to avoid the above problems. However, the compression equipment necessary for liquefaction of gaseous carbon dioxide, and the cryogenic equipment necessary for maintaining carbon dioxide in the liquid state, are also very expensive. Consequently, inorganic solvents do not provide a commercially attractive alternative.
It should, therefore, be appreciated that there is a need for a process that will yield crack-free porous monoliths having a sufficiently large pore size for ease of sintering, and that can be carried out below the critical pressure of the drying solvent so that the equipment costs can be reduced to make the process commercially attractive. The present invention fulfills this need.